In general, a storage system includes a memory device such as a disk array, and a control device that accesses the memory device in accordance with a request from a host apparatus. In recent years, there has been a demand for a highly extensible storage system that allows additional memory devices and/or control devices to be installed as appropriate after the storage system is initially introduced. Such a storage system allows a customer to save initial investment and to enhance access performance and storage capacity with small investment as the business expands.
As an example of a storage system, a shared storage distributed multimedia server system has been proposed as follows. The system includes element servers, a shared disk array including disk devices that store contents, and a shared channel network with which the element servers and the shared disk array are coupled and which enables a multi-initiator configuration. In response to a request from a client, each element server reads the content stored in the shared disk array and outputs the content to an information distribution network.
As an example of a disk array, the following disk array utilizing a loop-form interface has been proposed. The disk array has a configuration in which hard disk drives (HDDs) are coupled via individual port bypass circuits with a loop-form channel coupled by using Fibre Channel (FC) ports.
Related techniques are disclosed in, for example, Japanese Laid-open Patent Publication No. 10-134485 and Japanese Laid-open Patent Publication No. 2001-216206.
As an example of a storage system, a scale-out storage system is known in which additional installation may be made on a storage apparatus basis, where each storage apparatus includes a control device and one or more memory devices. In such a storage system, the storage capacity of the entire system is increased and access performance is improved by causing the control devices to access the memory devices in a distributed manner.
In addition, various processing may be performed through communications between the control devices of storage apparatuses. For instance, data may be transferred between the storage apparatuses so that the capacity and the access frequency may be balanced between the storage apparatuses. Data in a storage apparatus may be copied to another storage apparatus so that data redundancy may be increased. In addition, for one logical volume, physical storage areas controlled by a redundant array of inexpensive disks (RAID) may be distributed and assigned to a plurality of storage apparatuses. In this case, the availability against failures is improved not only on a memory device basis, but also on a storage apparatus basis.
As a method of allowing communication between control devices as described above, a method may be adopted in which the control devices are coupled with each other, for instance, via a relay apparatus. However, with this method, the communication load in the relay apparatus increases as the system size is increased due to additional installation of storage apparatuses. Therefore, there is a problem in that an expensive relay apparatus having high communication performance is desired in order to achieve a large-scale storage system. Alternatively, a method may be adopted in which the control devices are coupled with each other over a network. However, even with this method, the network traffic increases as the system size is increased, and thus a network with a high bandwidth has to be used.